This disclosure relates to capacitor films, and in particular to polyetherimide capacitor films.
Electrostatic film capacitors with high volumetric energy density, high operating temperature, and long lifetime are critical components for pulse-power, automotive, and industrial electronics. Capacitors are essentially energy-storing devices having two parallel conductive plates separated by a thin layer of an insulating (dielectric) film. When a voltage is applied across the plates, the electric field in the dielectric displaces electric charges, and thus stores energy. The amount of energy stored by a capacitor depends on the dielectric constant of the insulating material and the dimensions (total area and thickness) of the film, such that in order to maximize the total amount of energy that a capacitor can accumulate, the dielectric constant and breakdown voltage of the film are maximized, and the thickness of the film minimized. Because the physical characteristics of the dielectric material in the capacitor are the primary determining factors for the performance of a capacitor, improvements in one or more of the physical properties of the dielectric material in a capacitor can result in corresponding performance improvements in the capacitor component, usually resulting in performance and lifetime enhancements of the electronics system or product in which it is embedded.
Electrostatic film capacitors made from biaxially-oriented poly(propylene) (BOPP) have been used in applications requiring a low dissipation factor, high insulation resistance and low dielectric absorption, such as in electrical appliances, electronic equipment, oven and furnaces, refrigerators, automobiles, and home appliances. The low dielectric constant (Dk), which is about 2.2, and the maximum service temperature of about 100° C. limits the use of these capacitors in applications requiring high operating temperatures and/or high energy densities. Poly(ethylene terephthalate (PET) and poly(carbonate) (PC) films have a higher dielectric constant than BOPP films (about 3.0), but capacitors made from these films can only be used at operating temperatures as high as about 125° C.
Polyetherimide films manufactured by solvent casting can have a dielectric constant of about 3.2 and operating temperatures as high as about 200° C. The solvent-casting process requires use of solvents that increase the manufacturing cost of the films, as well as films that can have small amounts of solvent entrained therein. To remedy these deficiencies, extrusion has been proposed for the manufacture of polyetherimide films. It has been found, however, that extrusion can be wasteful, where 15 weight percent, 20 weight percent, or even more of the polyetherimide starting material fed to the extruder is not converted to films that are high enough quality for use in capacitors.
There accordingly remains a need in the art for new films and methods for their manufacture that can produce films of very high purity and with excellent electrical properties, in particular high breakdown strength and high dielectric constant. It would be a further advantage if such films could operate at high temperature. There remains a further need for efficient methods for producing such films that are amendable to industrial scale processes. It would be further advantage if such methods were environmentally friendly.